Nanofiltration (NF) is a pressure-driven technique that is gaining popularity due to its low consumption of energy, high water permeability and retention of multivalent ions as compared to the well-established reverse osmosis process [1, 2]. Such membranes have been researched for the application in many areas such as pre-treatment for the desalination process and have shown to be able to remove turbidity, microorganisms and dissolved salts [3].
A NF membrane usually consists of a thin active layer supported by a porous sublayer or substrate layer. This active layer plays the determining role in permeation and separation characteristics while the porous sublayer imparts the mechanical strength. There are many approaches to fabricate this active layer, namely:
(1) interfacial polymerization [6],
(2) layer-by-layer assembly [7, 8],
(3) chemical crosslinking [9] and
(4) UV grafting [10].
Among these approaches, UV grafting has been applied for years due to its advantages such as ease of operation and low cost [11, 12]. In addition, the fabrication via UV grafting produces an integral selective layer due to a strong chemical bond to the substrate which provides sufficient mechanical stability under relatively high operating pressure.
It has been known that polyethersulfone (PESU) can generate free radicals upon exposure to UV light due its photosensitive nature [13]. Thus, vinyl monomers in contact with free radicals can form a covalent bond with PESU.
The separation behaviour of NF membranes comprises size exclusion as well as electrostatic repulsion [4]. Thus, for the removal of cationic compounds, the use of a positively charged membrane is more effective than a negatively charged one. However, commercially available NF membranes are mostly negatively charged [5]. Hence, problem of the present invention is to provide positively charged NF membranes in order to improve performance in this area of separation.